Distinct ²⁹Si MAS NMR peaks from Si-Al permutation on neighboring T sites of unequal Si-O-T angles: Direct evidence from J-resolved experiment on K-cymrite (KAlSi ₃O ₈·H ₂O)

²⁹Si MAS NMR is a well-established tool for quantitative characterization of Si-Al distribution in aluminosilicates. In framework aluminosilicates, all tetrahedrally coordinated Si/Al (T) are linked via bridging oxygens to four other Si/Al (Q ⁴). Each crystallographically unique T site is commonly assumed to give at most five ²⁹Si NMR peaks with chemical shift spacing around 5 ppm, corresponding to five Si units each linked to nSi and (4 - n)Al (n = 0-4), abbreviated as Si(nSi) hereafter. Here we report a detailed one- and two-dimensional (2D) ²⁹Si NMR study on K-cymrite (KAlSi ₃O ₈·H ₂O), which possesses a double-layered structure with all Si/Al distributed in one crystallographically unique T site (Q ⁴). Contrary to general belief, more than five ²⁹Si MAS NMR peaks were resolved. These peaks are attributable, via 2D J-coupling mediated experiments, to Si(nSi)(n = 0-4), with two peaks each (∼2 ppm spacing) for n = 1-3. The latter corresponds to different permutations of the nSi(4 - n)Al over neighboring T sites of two distinct Si-O-T angles (139 and 180°). This suggests for the first time that the ²⁹Si chemical shift varies with Si-O-Si and Si-O-Al angles differently. Si-Al exchange over neighboring T sites of unequal Si-O-T angles is another possible cause for multiple ²⁹Si NMR peaks.